The blood groups

Blood group antigens exist on the surface of the red cell membrane (see also p. 4). There are numerous blood group systems encoded by genes on different chromosomes. They are highly variable in their polymorphism and clinical significance.

The most important blood group is the ABO system. The genes encoding the ABO antigens are located on chromosome 9 and are inherited in an autosomal dominant fashion. Each antigen is a sugar residue made by a specific glycosyl transferase. The ABO system is crucial in clinical blood transfusion as there are naturally occurring IgM antibodies in the serum targeted against the non-present ABO antigens (Table 41.1). These antibodies necessitate the use of ABO ‘compatible’ blood for transfusion. For example, the administration of incompatible group A blood to a group B patient would engender a potentially fatal haemolytic transfusion reaction due to the destruction of the donor’s group A red cells by the recipient’s anti-A antibody.

In other blood group systems ‘naturally occurring’ antibodies are rare. However, ‘immune antibodies’, usually of IgG type, may be induced by transfusion of blood expressing different blood group antigens or maternal exposure to fetal red cell antigens. Where such immune antibodies are present, transfused blood must be matched for the relevant blood group system in addition to ABO. Maternal formation of immune antibodies against antigens of the Rhesus (Rh) blood group system, particularly the strongest antigen D, accounts for most cases of haemolytic disease of the newborn (p. 90).

The testing of blood